There are various types of check valves based on the swing system, butterfly system, lift system, or cone system. Of these check valves, a check valve based on the swing system has, as components of the basic configuration, a valve box having a flow path, and a valve body rotatably mounted on a valve shaft and functioning to open or close the flow path of the valve box. A document disclosing this type of check valve is, for instance, Japanese Patent Laid-Open Publication No. HEI 7-145870. This document discloses a check valve based on the swing system in which a valve body is energized and turned by a spring to the valve-closing direction, and the spring is wound around a spring guide bar provided at a downstream position off from the valve shaft and also in parallel to the valve shaft. With this configuration, the energizing force of the spring acting to the valve body becomes stronger, which ensures quick opening and closing operations of the valve body and prevention of generation of water hammer.
In the check valve based on the conventional technology as described above, when a fluid flows in the reverse direction, the spring energizing the valve body to turn in the valve-closing direction prevents the valve from being quickly closed, but it is required to provide the spring guide bar as described above, a bearing guide and the like as described above to prevent water hammer, which complicates a structure inside the valve box and requires facility cost, and therefore the cost becomes disadvantageously high. There has been proposed a valve body having a small valve hole provided thereon, but in this case it is required to additionally provide a small valve body for opening and closing the small valve hole, and the valve body is easily broken when used for a long time, which is disadvantageous. Further there has been proposed the configuration in which a bypass conduit communicating an upstream side of the valve box flow path to the downstream side thereof is provided outside the valve box, but in this case a certain space is necessary outside the valve box, which sometimes causes troubles in installation of the valve.